The present invention relates to ink drop printing apparatus and more particularly to an improved arrangement for charging ink drops.
In ink drop printing apparatus, wherein charged ink drops are projected through an electric field in order to be deflected in accordance with the charge on the drop and thereafter fall on paper to form characters or symbols, in order to improve the resulting printing, it has been necessary to compensate both for aerodynamic effects and for the effects of the charges on the drops on each other. The aerodynamic effect is the retardation in the speed of the leading drop in a stream of drops, the leading drop being slowed down by the drag of the air which it must part. The succeeding drops are not affected as much as the first drop. The effect due to a charge on a drop arises, in a stream of drops, where, for example, a first drop in a stream is repelled by the charge on the following drop or drops and is caused to speed up in its passage through the air. The first drop in a stream of two drops will not be repelled nearly as much as the first drop in a stream of more than two drops. The last drop in a stream of drops is effectively pushed away from the stream of drops by the interaction of its charge and the field caused by the charge on the adjacent drop in the stream of drops.
Another problem that arises, when correcting for the repulsive force between the last drop and the drop before the last drop in a stream of charged drops by means of lowering the charge amplitude on the last drop, is that the charged ink drop before the last drop is deflected away from the stream by the interaction of its charge and the field resulting from charges on the adjacent drops in the stream of drops.
Still another problem that arises, which needs solution if excellent ink drop printing is desired, is that the amount of deflection that a single charged drop receives in passing through an electric field is greater than the amount of deflection that a drop, in a stream of drops, which has the same amplitude of charge as the single drop, receives in passing through the same electric field. This is caused by the relatively slower velocity with which a single drop passes through the electric field than does a stream of drops.
Yet another adverse effect has been noted which requires solution. An uncharged drop, not to be used in writing, which follows a stream of charged drops, has a charge induced therein, as a result of the charge on the last charged drop, which is of opposite polarity and of smaller amplitude than the charge on the last charged drop. As a result, this drop may be deflected enough in passing through the electric field, so that it will not be intercepted by the waste catcher and will instead cause ink to accumulate on the deflection plate.
A number of patents have been issued which teach how to provide charge compensation to take care of the effects of the charge of adjacent charged drops. These may be found in U.S. Pat. No. 3,631,511 to Keur, et al, and in U.S. Pat. No. 3,789,422, and U.S. Pat. No. 3,827,057. However, these patents do not teach how one may select the leading drop in a stream of only two drops and in a stream of two or more drops, and the trailing drop in a drop stream for charge compensation, nor do they teach how one may select a single charged drop for charge compensation, or how one may compensate an uncharged drop which immediately follows a charged drop for the effects of the charged drop thereon.